1. Field of the Invention
The present invention relates to a display device that conducts a display operation using four or more primary colors.
2. Description of the Related Art
Various types of display devices are currently used in a variety of applications. In a general display device, one pixel consists of three subpixels respectively representing red, green and blue, which are the three primary colors of light, thereby conducting a display operation in colors.
A conventional display device, however, can reproduce colors that fall within only a narrow range (which is usually called a “color reproduction range”), which is a problem. FIG. 13 shows the color reproduction range of a conventional display device that conducts a display operation using the three primary colors. Specifically, FIG. 13 shows an xy chromaticity diagram according to the XYZ color system, in which the triangle, formed by the three points corresponding to the three primary colors of red, green and blue, represents the color reproduction range. Also plotted by symbols “X”, in FIG. 13 are the surface colors of various objects existing in nature, which were disclosed in M. R. Pointer, “The Gamut of Real Surface Colors”, Color Research and Application, Vol. 5, No. 3, pp. 145-155 (1980). As can be seen from FIG. 13, there are some object colors that do not fall within the color reproduction range, and therefore, a display device that conducts a display operation using the three primary colors cannot reproduce some object colors. It should be noted that the color reproduction range of a display device is often represented as a ratio to a reference color reproduction range that was defined by some association such as EBU (European Broadcasting Union) or NTSC (National Television System Committee). And such a ratio is called an “EBU ratio” or an “NTSC ratio”, for example.
As described above, since a conventional display device has too narrow a color reproduction range to reproduce some colors (i.e., the colors outside of the color reproduction range) even if the image capture device can record colors in a broad range including those non-reproducible colors. That is why at any stage of signal processing to be done by either the image capture device or the display device, those colors outside of the color reproduction range of the display device need to be corrected and converted into colors that fall within that color reproduction range. Hereinafter, it will be described exactly how that signal processing can be done.
For example, a normal TV program is broadcast by transmitting a YCrCb signal including a luminance signal Y and two chrominance signals Cr and Cb. The YCrCb signal that has been transmitted is converted into an RGB signal including components that represent the respective luminances (i.e., grayscales) of red, green and blue, and the display device conducts a display operation based on this RGB signal.
If every signal level available were used, the YCrCb signal itself could represent those colors outside of the color reproduction range defined by the EBU standard. However, if the YCrCb signal representing those colors that do not comply with the EBU standard were simply converted into an RGB signal, at least one of the red, green and blue components would have a negative level. The conventional display device cannot reproduce such colors that are not compliant with the EBU standard. That is why if an RGB signal including such a component with the negative level is input, those colors can be corrected into colors that are compliant with the EBU standard that can be reproduced by the display device by regarding the negative level component as zero (which is called “clipping”).
For example, if a YCrCb signal representing the color cyan that is not compliant with the EBU standard as shown in FIG. 16 (which is indicated by the point R−in FIG. 16) were converted into an RGB signal, then the red component would have a negative level, thus generating a signal representing a so-called “negative grayscale”. The conventional display device cannot reproduce such a color cyan as it is, and therefore, presents such a color after correcting it into a color cyan compliant with the EBU standard (which is indicated by the point R0 in FIG. 16) by clipping the red component and regarding it to be zero. By performing such correction processing, even the conventional display device can also reproduce such a color. However, as the reproduced color is a corrected one, the natural object color cannot still be reproduced, which is a problem.
Meanwhile, in order to broaden the color reproduction range of display devices, a technique that uses an increased number of primary colors for display purposes has recently been proposed.
For example, PCT International Application Japanese National-Phase Publication No. 2004-529396 discloses a liquid crystal display device 800 in which one pixel P consists of six subpixels representing the colors red, green, blue, yellow, cyan and magenta, respectively, as shown in FIG. 14. The color reproduction range of such a liquid crystal display device 800 is shown in FIG. 15. As shown in FIG. 15, the color reproduction range, represented by a hexagon of which the vertices correspond to those six primary colors, covers almost all object colors. By increasing the number of primary colors for use in display in this manner, the color reproduction range can be broadened. Such display devices that conduct a display operation using four or more primary colors will be collectively referred to herein as “multi-primary-color display devices”.
However, the conventional broadcasting standards suppose the use of a display device that conducts a display operation using the three primary colors. That is why even by simply adopting a multi-primary-color display device such as the one disclosed in PCT International Application Japanese National-Phase Publication No. 2004-529396, that broad color reproduction range of the multi-primary-color display device cannot be made full use of, and a display operation cannot be conducted with a sufficiently broad color reproduction range, either.
For example, if a signal representing a negative grayscale were clipped as in the conventional display device, then the color reproduction range realized would be almost no different from the one realized by the conventional device after all. And nobody has ever established a technique for getting colors to be represented by a video signal, including a negative level component, reproduced by a multi-primary-color display device with fidelity.